This invention is generally directed to the use of amorphous silicon compositions as electrophotographic imaging members, and more specifically, the present invention is directed to photoresponsive layered imaging devices comprised of amorphous silicon overcoated with insulating protective layers. In one embodiment of the present invention, there are provided overcoated layered photoresponsive devices containing two amorphous silicon layers. These devices can be incorporated into an electrophotographic imaging system, particularly xerographic imaging systems, wherein the latent electrostatic images which are formed, can be developed into images of high quality, and excellent resolution.
Electrostatographic imaging systems, particularly xerographic imaging systems are well known, and are extensively described in the prior art. In these systems generally, a photoresponsive or photoconductor material is selected for forming the latent electrostatic image thereon. This photoreceptor is generally comprised of a conductive substrate containing on its surface a layer of photoconductive material, and in many instances, a thin barrier layer is situated between the substrate and the photoconductive layer to prevent charge injection from the substrate, which injection would adversely affect the quality of the resulting image. Examples of known useful photoconductive materials include amorphous selenium, alloys of selenium, such as selenium-tellurium, selenium-arsenic, and the like. Additionally, there can be selected as the photoresponsive imaging member various organic photoconductive materials, including, for example, complexes of trinitrofluorenone and polyvinylcarbazole. Recently, there has been disclosed multilayered organic photoresponsive devices containing a charge transport layer comprised of for example substituted diamines dispersed in an inactive resinous binder, and a photogenerating layer, reference U.S. Pat. No. 4,265,990 the disclosure of which is totally incorporated herein by reference. Examples of charge transport layers include various diamines, while examples of photogenerating layers include trigonal selenium, metal and metal-free phthalocyanines, vanadyl phthalocyanines, squaraine compositions, and the like.
Many other patents are in existence describing photoresponsive devices containing generating substances, such as U.S. Pat. No. 3,041,167, which discloses an overcoated imaging member containing a conductive substrate, a photoconductive layer, and an overcoating layer of an electrically insulating polymeric material. This member is functional in an electrophotographic method by, for example, initially charging the photoresponsive device with an electrostatic charge of a first polarity, imagewise exposing enabling the formation of an electrostatic latent image thereon, and subsequently developing the resulting image. Prior to each succeeding imaging cycle, the photoconductive member can be charged with an electrostatic charge of a second opposite polarity, and sufficient additional charges of this polarity are applied so as to create across the member a net electrical field. Simultaneously, mobile charges of the first polarity are created in the photoconductive layer by applying an electrical potential to the conductive substrate. The imaging potential which is developed to form the visible image is present across the photoconductive layer and the overcoating layer.
There is also disclosed in a copending application, U.S. Ser. No. 524,801, electrostatographic imaging devices containing compensated amorphous silicon compositions, wherein there is simultaneously present in the amorphous silicon dopant materials of boron and phosphorous. More specifically there is disclosed in the copending application a photoresponsive device comprised of a supporting substrate, and an amorphous silicon composition containing from about 25 parts per million by weight to about 1 weight percent of boron, compensated with from about 25 parts per million by weight to about 1 weight percent of phosphorous.
Additionally amorphous silicon photoconductors are known, thus for example there is disclosed in U.S. Pat. No. 4,265,991 an electrophotographic photosensitive member containing a substrate, a barrier layer, and a photoconductive overlayer of amorphous silicon containing 10 to 40 atomic percent of hydrogen and having a thickness of 5 to 80 microns. Further described in this patent are several processes for preparing amorphous silicon. In one process embodiment, there is prepared an electrophotographic sensitive member by heating the member in a chamber to a temperature of 50.degree. C. to 350.degree. C., introducing a gas containing a hydrogen atom into the chamber, causing an electrical discharge by electric energy to ionize the gas, in the space of the chamber in which a silicon compound is present, followed by depositing amorphous silicon on an electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second, thereby resulting in an amorphous silicon photoconductive layer of a predetermined thickness. While the amorphous silicon device described in this patent is photosensitive, after a minimum number of imaging cycles, less than about 10, for example, unacceptable low quality images of poor resolution, with many deletions, result. With further cycling, that is, subsequent to 10 imaging cycles and after 100 imaging cycles, the image quality continues to deteriorate often until images are partially deleted. Accordingly, while the amorphous silicon photoresponsive device of the '991 patent is useful, its selection as a commercial device which can be used functional for a number of imaging cycles is not readily achievable.
While it is not desired to be limited to theory, it is believed that the degradation of the electrophotographic performance of amorphous silicon is caused by the sensitivity of the surface of the silicon device to physical and chemical alterations, including abrasion, scratching, and exposure to a corona atomsphere, especially at high humidities. These sensitivities create fundamental limitations for the practical use of devices wherein the exposed surface contains substantially amorphous silicon. This problem can be minimized by encapsulating the amorphous silicon with a chemically passive, hard overcoating layer of amorphous silicon nitride, amorphous silicon carbide, or amorphous carbon, however when these devices are incorporated into xerographic imaging systems there results image blurring and very rapid image deletion in a few imaging cycles, typically less than about 10. With overcoated silicon devices, poor image quality with cycling is caused by an increase in the surface conductivity of the underlying amorphous silicon layer, rather than by abrasion or chemical interactions with the photosensitive surface as occurs with amorphous silicon containing no protective overcoating layer, which conductivity increase is induced by the electric field existing at the surface of the overcoated device, similar to that resulting from the field effect in well-known metal-insulator-semiconductor devices. The induced surface conductivity causes a lateral spreading of the photogenerated charges in the electric field fringe fields associated with line or edge images projected on the photoreceptor surface, thus causing undesirable image blurring and image deletion.
The existence of a field effect phenomena in amorphous silicon is well known, as this material functions as an extrinsic amorphous semiconductor, that is, a semi-conductor whose conductivity can be substantially modified by impurity doping and by electric fields. In contrast, the conductivities of many other photoreceptor materials, such as those based on chalcogenides, will not be significantly modified by either impurity doping or electric fields.
The above disadvantages are substantially eliminated with the photoresponsive device of the present invention, accordingly for example image deletion, and image blurring, is not observed in the photoconductive devices of the present invention comprised of overcoated amorphous silicon compositions with a thin trapping layer situated between the amorphous silicon composition and the insulating overcoating layer. Essentially this device is a multilayered structure of such design as to minimize or eliminate the induced lateral conductivity and the image blurring and deletion caused thereby. More specifically, the present invention provides substantially hydrogenated amorphous silicon compositions and device structures incorporating trapping layers, which function to prevent image resolution loss. By trapping, which term is well known in the semiconductor arts, is meant the immobilization of a charge carrier. This spatial immobilization is provided by a trapping site, the existence of which is caused and controlled by extrinsic means such as the disruption of native atomic bonds or the incorporation of dopants therein. Image deletion, and image blurring, is not observed in the photoconductive devices of the present invention comprised of overcoated amorphous silicon compositions with a thin trapping layer situated between the amorphous silicon composition and the insulating overcoating layer.
Thus, while amorphous silicon based devices with and without, the trapping layers of the present invention are substantially electrically similar, that is, they are both photosensitive, can be charged to high electric fields, and have good carrier range, they differ significantly in their image capabilities in that after 10 imaging cycles, images formed with amorphous silicon photoconductors which are overcoated to passify the surface, but which do not incorporate a trapping layer begin to deteriorate rapidly as disclosed hereinbefore. There thus continues to be a need for improved photoconductor materials, particularly photoconductive devices containing amorphous silicon which can be repeatedly used in a number of imaging cycles without deterioration therefrom. Additionally, there continues to be a need for improved layered imaging members containing amorphous silicon insulating overcoated multilayered structures which are designed to be humidity insensitive, and are not adversely affected by the electrical consequences resulting from scratching and abrasion. Further there continues to be a need for improved photoresponsive devices containing charge carrier trapping layers, which devices can be prepared with a minimum number of processing steps, and wherein the layers are sufficiently adhered to one another to allow the continuous use of such devices in repetitive imaging and printing systems. Moreover, there continues to be a need for photoresponsive devices containing charge carrier trapping layers, wherein the incorporation of these layers in such devices do not adversely affect the electrical and photoconductive characteristics thereof; and wherein the xerographic imaging capabilities of the devices are significantly improved. Also, there continues to be a need for amorphous silicon materials which can be selected for incorporation into an electrophotographic imaging system, wherein such materials are not sensitive to humidity and corona ions generated by the charging apparatus, thereby allowing such a material to be useful over a substantial number of imaging cycles without causing a degradation in image quality, and specifically, without resulting in blurring of the images produced. There further continues to be a need for amorphous multilayered silicon-based devices which do not incorporate high dopant concentrations thereby causing undesirable cross contamination effects during sequential layer deposition. Finally, there continues to be a need for amorphous silicon multilayered devices where the electrical performance thereof is critically depend on the details of the fabrication process which is used to form the interfaces between the various layers.